Perovskite-type oxides of the ABO.sub.3 type (where the element A is Pb, Ca, Sr or La; and the element B is Ti or Zr) are conventionally produced by the following methods:
(1) mixing the powders of the oxides of constituent elements and heating the mixture to high temperatures to cause a solid-phase reaction;
(2) adding oxalic acid dropwise to an aqueous solution containing the ions of constituent elements, coprecipitating the constituent elements in the form of their oxalic acid salts, and subjecting the coprecipitated oxalic acid salts to thermolysis (see, for example, U.S. Pat. No. 3,352,632);
(3) hydrolyzing the mixture of alkoxides of constituent elements to form a precipitate and subjecting the coprecipitated hydrolyzate to thermolysis (see, for example, Japanese Laid-Open Patent Application No. 86022/85); and
(4) a multi-stage wet process in which lead hydroxide or zirconium hydroxide is preliminarily synthesized using aqueous ammonia and a solution of titanium tetrachlorides is added thereto, followed by precipitation with aqueous ammonia (see, for example, Japanese Laid-Open Patent Application No. 106456/86).
However, these methods have one or more problems and are far from being satisfactory for commercial purposes. The major problem of the first method concerns a manufacturing process in that the solid-phase reaction involved. requires high temperatures and prolonged periods. In addition, the powder produced by this method is defective in that it is difficult to sinter unless elevated temperatures or a sintering accelerator is employed. In the second method, the oxalic acid salts of the ingredients have different solubilities in water which is used as a coprecipitation medium and hence it is difficult to exactly control the proportion in which the respective ingredients are to be coprecipitated in such a way that a single-phase composition will be produced. The third method has the advantage that a homogeneous product of high purity is produced but its production is not easy to achieve since the respective ingredients are used in the form of alkoxides. The fourth method which relies upon a multi-stage wet process can be performed using inexpensive materials but the calcined product must be crushed before sintering.
Two methods have been proposed to improve the second process involving the use of oxalic acid. They take advantage of the fact that oxalic acid is soluble in ethanol whereas the oxalic acid salts of elements A and B are both insoluble in ethanol. According to the first approach, the ion of element A and Ti ion are reacted with oxalic acid in ethanol so that the two ions are coprecipitated as oxalic acid salts (Japanese Laid-Open Patent Application No. 39722/84). In the second approach, the ion of element A and Zr ion or (Zr+Ti) ion are coprecipitated as oxalic acid salts (Japanese Laid-Open Patent Application No. 131505/84). As a result, a precipitate of a desired composition that has high purity and a uniform grain size (i.e., a precursor of the desired perovskite-type oxide) is formed. This precipitate is subjected to thermolysis so as to produce a fine powder of ATiO.sub.3, AZrO.sub.3 or A(Zr,Ti)O.sub.3 that is active and is easily sinterable. In these improved methods, the ion of element A is used as a solution of a nitric acid salt of element A in water or ethanol-containing water.
It is generally held that Ti and Zr ions are preferably used as a solution of titanium oxynitrate or zirconium oxynitrate in water or ethanol-containing water. It has been pointed out that if chlorides are used as sources of the supply of these ions, chloride ions tend to remain in the coprecipitate; even if this coprecipitate is calcined at elevated temperatures, the residual chloride compounds still remain in the calcined product (namely, the desired oxide) and cause adverse effects mainly on its sinterability. It has also been pointed out that if Pb.sup.2+ is used as the ion of an element A, undesirable insoluble lead chloride will be formed in an aqueous solution of mixed ions.
In the two improved versions of the second method, Ti ions (from titanium oxynitrate) or Zr ions (from zirconium oxynitrate) and the ions of an element A are reacted with oxalic acid in the presence of ethanol and the resulting coprecipitate of oxalic acid salts is filtered, dried and crushed into particles, which are calcined at a temperature between 700.degree. and 1,000.degree. C. where calcination of the coprecipitate is completed and no change in weight will occur. Although these procedures enable the production of a desired perovskite-type oxide, the proposed methods require that the calcined product be crushed again into particles, which are mixed together before they are molded and sintered at 1000.degree. to 1400.degree. C.
The reason for the need to crush the calcined product into particles and to mix them together is that the fine grains of perovskite-type oxide have fused together to form agglomerates during calcination. The step of recrushing and remixing not only increases the manufacturing cost but also reduces the reliability of the final product by letting impurities get into the calcined product. This step causes another problem associated with the characteristics of the powder perovskite-type oxide.
An active research is underway to produce highly flexible piezoelectric and dielectric films from composites of the perovskite-type oxide powder with other materials such as poly(vinylidene fluoride) resins, polyoxymethylene resins, or nitrile-butadiene rubbers. In order to produce satisfactory products, the fine powder of perovskite-type oxide must satisfy the requirements that they have a uniform grain size distribution, be free from crystal distortion and be easy to disperse into organic compounds. However, it is known that the fine powder obtained by the step of recrushing and remixing has crystal distortions induced in it and is unable to display any of the anticipated performances.
Another point that should be noted here is that although fine ceramic grains must be uniformly dispersed in a substrate with a thickness of not more than 10 .mu.m, preferably 1 to 5 .mu.m, to make a dielectric film, the crushed particles have such a broad grain size distribution that the desired film reliability cannot be ensured.
The present inventors previously conducted extensive studies in order to unravel the mechanism by which the fine particles of calcined product would fuse together to form agglomerates. As a results, the present inventors found that chloride ions remaining in small amounts in an aqueous solution of the mixed ions from starting materials were responsible for the agglomeration of fine particles during calcination and that this phenomenon could be retarded by reducing the concentration of chloride ions below a certain level. Based on these findings, the present inventors and Yamamura filed a patent application which was later published as Japanese Laid-Open Patent Application No. 174116/86.
Following the filing of this Laid-Open patent application, the present inventors proposed the following in Japanese Laid-Open Patent Application No. 211516/86: if the precipitate of a precursor is synthesized under such conditions that the concentration of the ions of element A in the aqueous solution is in the range of 0.2 to 1 mol/l, with ethanol being used in 0.5 to 4 volumes per volume of the aqueous solution, a powder having the same characteristics as those of the product shown in Japanese Laid-Open Patent Application No. 174116/86 is synthesized and with the cost of its production being significantly reduced in account of the dramatic decrease in the amount of the ethanol used.
The present inventors made another proposal in Japanese Laid-Open Patent Application No. 211519/86 by showing that ethanol may be replaced by isopropanol or normal propanol to obtain a perovskite-type oxide powder having comparable performance. This proposal offered the potential to realize a further reduction in the manufacturing cost of a perovskite-type oxide powder.
The present inventors also proposed in Japanese Patent Laid-Open Application No. 251517/86 that the ions of element B that partly dissolve in the aqueous solution containing ethanol or isopropanol be precipitated again by blowing dry ammonia into that solution which has been thoroughly agitated to disperse the precipitate after completion of the reaction of precipitate formation. According to this method, the loss of the ion of element B can be reduced while the proportions of the ions of elements A and B in the precipitate are precisely controlled.
In their continued research, the present inventors conducted a series of experiments in which an aqueous solution containing lead nitrate and titanium oxynitrate was subjected to the reaction for the formation of the precipitate of oxalic acid: salts (i.e., precursor of the perovskite-type oxide) using alcohols having 4 to 15 carbon atoms. It was found that the Ti/Pb atomic ratio set for the feed materials was essentially reproduced in PbTiO.sub.3 that was obtained by calcining the precipitate. This result was quite surprising in that it could not have been attained with ethanol or propanol. A proposal based on this finding was disclosed in Japanese Laid-Open Patent Application No. 251518/86.
The present inventors continued their research on how to control the proportions of constituent elements in oxides of the ABO.sub.3 type in oxalic salts coprecipitation method.
One of the observations found as a result of these efforts was that a desired oxide of the ABO.sub.3 type could be produced quantitatively and consistently over time by controlling the constituent elements in such proportions that the oxalic acid (precipitating agent) to Ti in TiO(NO.sub.3).sub.2 was adjusted to 1/2 (mol/mol), rather than the conventionally established value of 1/1 (mol/mol). On the basis of this finding, the present inventors disclosed in Japanese Laid-Open Patent Application No. 72523/87 that by employing this novel ratio, the fine grains of an oxide of the ABO.sub.3 type could be consistently synthesized from a combination of Pb (element A) and at least one of Ti and Zr (element B) in an oxalic acid/ethanol system. The present inventors also disclosed that similar results could be attained by employing an oxalic acid/water system (Japanese Laid-Open Patent Application No. 72524/87).
In the specifications of the above-mentioned prior inventions, the present inventors showed that the fine particles of an oxide of the ABO.sub.3 type could be consistently produced by applying an unconventional stoichiometric ratio to the precipitate of oxalic acid salts formed from titanium oxynitrate and zirconium oxynitrate. However, as will be understood from the fact that only Pb is mentioned as element A, Ca, Sr, Ba, La, etc. are unsuitable for use as element A since their oxalates will partly dissolve into the mother liquor under the conditions employed in these two prior inventions and are incapable of yielding an intended oxide of the ABO.sub.3 type in a quantitative manner.
In the prior art techniques, alcohols were used in order to prevent the ions of elements A and B from dissolving in an aqueous solution. This method was effective in completely inhibiting the dissolution of Pb, Zr and Ti oxalates in a range rendered acidic with nitric acid. However, as illustrated by the following reaction schemes, conversion of nitrates to oxalates involves the formation of nitric acid in a large amount: EQU Pb(NO.sub.3).sub.2 +H.sub.2 C.sub.2 O.sub.4 .fwdarw.PbC.sub.2 O.sub.4 +2HNO.sub.3 EQU Ca(NO.sub.3).sub.2 +H.sub.2 C.sub.2 O.sub.4 .fwdarw.CaC.sub.2 O.sub.4 +2HNO.sub.3 EQU 2TiO(NO.sub.3).sub.2 +H.sub.2 C.sub.2 O.sub.4 .fwdarw.(TiO).sub.2 C.sub.2 O.sub.4 (NO.sub.3).sub.2 +2HNO.sub.3.
Oxalates other than PbC.sub.2 O.sub.4 such as CaC.sub.2 O.sub.4, SrC.sub.2 O.sub.4, BaC.sub.2 O.sub.4 and La.sub.2 (C.sub.2 O.sub.4).sub.3 that were used to form the precipitate of a precursor containing element A were exposed to a strongly acidic atmosphere created by the presence of a large amount of nitric acid in mother liquor. Since the dissolution inhibiting action of ethanol and other alcohols was not fully exhibited under the strongly acidic condition, Ca ions could not be completely prevented from dissolving into the mother liquor. The results were the same with other non-Pb ions (i.e., Sr, Ba and La) and it was impossible to control exactly the ratio of existence of element A to element B in the finally obtained oxide of the ABO.sub.3 type.
However, Ca, Sr and Ba are elements that are indispensable to the synthesis of ceramics such as CaTiO.sub.3, SrTiO.sub.3, BaTiO.sub.3, (PbCa)TiO.sub.3, (PbSr)TiO.sub.3 and (PbBa)TiO.sub.3, and La is important as an element to be used in the synthesis of light-transmitting piezoelectric ceramics generally referred to as PLZT.
The present inventors continued their studies in order to develop a process by which not only perovskite-type oxides of the ABO.sub.3 type containing Pb as element A but also those having Pb substituent at site A with varying proportions of other metal ions such as Ca, Sr, Ba and La can be synthesized by a chemical synthesis reaction using the novel stoichiometric ratio disclosed in Japanese Laid-Open Patent Application Nos. 72523/87 and 72524/87. Special emphasis was placed on the efforts to seek a method for producing a fine-grained oxide of the ABO.sub.3 type by means of quantitatively precipitating not only Pb but also Ca, Sr or La as element A.